1. Field of the Invention
The present invention relates in general to a tappet for use between a push rod and a cam in an internal combustion engine and more particularly to a method of forming a tappet of the kind having a ceramic seat plate for contact with a cam.
2. Description of the Prior Art
With a view to improving the wear resistance of a tappet and thereby meeting the demand for a higher output of an engine, there has been proposed a tappet consisting of a tappet main body made of a single metallic material and a ceramic disk or seat plate brazed to an end of the tappet main body. The tappet main body is made of such steel that can be transformed to martensite by air cooling, such as SNCM 630 (nickel-chromium-molybdenum steel according to Japanese Industrial Standards) and SKC 24 (hollow drill steel according to Japanese Industrial Standards), so that it can be hardened by making use of the heat for brazing.
By the combination of metal and ceramics, the prior art tappet aims at attaining an improved durability with a reduced cost, i.e., aims at improving the durability by utilizing an excellent wear resistance of ceramics while lowering the cost by utilizing the low machining cost and low material cost of the metal.
However, the metal employed for the prior art tappet needs to have such a property that it can be easily brazed to the ceramic seat plate and be hardened by making use of the heat for brazing, and is therefore limited to an expensive steel such as SNCM 630, etc. Such a steel however is poor in machinability. Thus, when the entire of the tappet main body is made of such an expensive steel, a high material cost and high machining cost inevitably result. Accordingly, the prior art tappet is disadvantageous from the cost point of view.